At the suggestion of one of those trainer tips on The Biggest Loser, I chose to try out using an exercise ball as my office chair...
I've received a few odd looks from colleagues, but they are intrigued. Using an exercise ball apparently helps you by forcing you to keep your posture and you work a lot of core abdominal muscles all through out the day. Not a major contributor to burning calories, but more than sitting in a chair. Will see how this works out the rest of the week.